1. Field of the Invention
This invention relates generally to modem communications and, more particularly, to a software modem with privileged mode decryption of control codes.
2. Description of the Related Art
In recent years cellular telephones have become increasingly popular. A cellular telephone is one example of what is referred to as a “mobile station” or “mobile terminal.” A mobile station can take on various forms other than a cellular telephone, including a computer (e.g., a notebook computer) with mobile communication capabilities.
Telecommunications services are provided between a cellular telecommunications network and a mobile station over an air interface, e.g., over radio frequencies. Typically, each subscriber having a mobile station is assigned a unique International Mobile Subscriber Identity (IMSI). At any moment, an active mobile station may be in communication over the air interface with one or more base stations. The base stations are, in turn, managed by base station controllers, also known as radio network controllers. A base station controller together with its base stations comprise a base station system. The base station controllers of a base station system are connected via control nodes to a core telecommunications network, such as the publicly switched telephone network (PSTN). One type of standardized mobile telecommunications scheme is the Global System for Mobile communications (GSM). GSM includes standards that specify functions and interfaces for various types of services. GSM systems may be used for transmitting both voice and data signals.
A particular base station may be shared among multiple mobile stations. Because the radio spectrum is a limited resource, the bandwidth is divided using combination of Time-Division and Frequency-Division Multiple Access (TDMA/FDMA). FDMA involves dividing the maximum frequency bandwidth (e.g., 25 MHz) into 124 carrier frequencies spaced 200 kHz apart. A particular base station may be assigned one or more carrier frequencies. Each carrier frequency is, in turn, divided into time slots. During an active session between the base station and the mobile station, the base station assigns the mobile unit a frequency, a power level, and a time slot for upstream transmissions from the mobile station to the base station. The base station also communicates a particular frequency and time slot for downstream transmissions from the base station destined for the mobile station.
The fundamental unit of time defined in GSM is referred to as a burst period, which lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which is the basic unit for the definition of logical channels. One physical channel is defined as one burst period per frame. Individual channels are defined by the number and position of their corresponding burst periods.
GSM frames, each frame having 8 burst periods, are grouped into superframes (e.g., groups of 51 frames) that include both traffic (i.e., voice or data signals) and control information. The control information is conveyed over common channels defined in the superframe structure. Common channels can be accessed both by idle mode and dedicated mode mobile stations. The common channels are used by idle mode mobile stations to exchange signaling information for changing to dedicated mode in response to incoming or outgoing calls. Mobile stations already in the dedicated mode monitor the surrounding base stations for handover and other information.
The common channels include:                a Broadcast Control Channel (BCCH) used to continually broadcasts information including the base station identity, frequency allocations, and frequency-hopping sequences;        a Frequency Correction Channel (FCCH) and Synchronization Channel (SCH) used to synchronize the mobile station to the time slot structure of a cell by defining the boundaries of burst periods, and the time slot numbering (i.e., every cell in a GSM network broadcasts exactly one FCCH and one SCH, which are, by definition, sent on time slot number 0 within a TDMA frame);        a Random Access Channel (RACH) used by the mobile station to request access to the network;        a Paging Channel (PCH) used to alert the mobile station of an incoming call; and        an Access Grant Channel (AGCH) used to allocate a Stand-alone Dedicated Control Channel (SDCCH) to a mobile station for signaling (i.e., to obtain a dedicated channel) following a request on the RACH.        
For security reasons, GSM data is transmitted in an encrypted form. Because a wireless medium can be accessed by anyone, authentication is a significant element of a mobile network. Authentication involves both the mobile station and the base station. A Subscriber Identification Module (SIM) card is installed in each mobile station. Each subscriber is assigned a secret key. One copy of the secret key is stored in the SIM card, and another copy is stored in a protected database on the communications network that may be accessed by the base station. During an authentication event, the base station generates a random number that it sends to the mobile station. The mobile station uses a random number, in conjunction with the secret key and a ciphering algorithm (e.g., A3), to generate a signed response that is sent back to the base station. If the signed response sent by the mobile station matches the one calculated by network, the subscriber is authenticated. The base station encrypts data transmitted to the mobile station using the secret key. Similarly, the mobile station encrypts data it transmits to the base station using the secret key. After a transmission received by the mobile station is decrypted, various control information, including the assigned power level, frequency, and time slot for a particular mobile station may be determined by the mobile station.
Generally, communication systems are described in terms of layers. The first layer, responsible for the actual transmission of a data carrying signal across the transmission medium, is referred to as the physical layer (PHY). The physical layer groups digital data and generates a modulated waveform based on the data in accordance with the particular transmission scheme. In GSM, the physical layer generates the transmission waveform and transmits during the assigned transmit time slot of the mobile station. Similarly, the receiving portion of the physical layer identifies data destined for the mobile station during the assigned receipt time slot.
The second layer, referred to as a protocol layer, processes digital data received by the physical layer to identify information contained therein. For example, in a GSM system, decryption of the data is a protocol layer function. Notice that changes in the operating parameters of the physical layer are identified only after decryption and processing by the protocol layer. Although this particular interdependency does not generally cause a problem in a purely hardware implementation, it may cause a problem when all or portions of the protocol layer are implemented in software.
Certain computer systems, especially portable notebook computers, may be equipped with wireless modems. One trend in modem technology involves the use of software modems that implement some of the real-time functions of traditional hardware modems using software routines. Because the hardware complexity of a software modem is less than a hardware counterpart, it is generally less expensive as well as more flexible. For example, the protocol layer decryption and processing may be implemented partially or entirely with software.
Software systems, such as PC systems, run interface control software in operating systems environments as software drivers. These drivers are responsible for communicating to the hardware devices and operate at a privileged level in the operating system. Other software applications are precluded from affecting the drivers. However, because drivers are not protected from other drivers, a variety of problems can occur that might affect the operation of a driver, such as by corrupting its operation. These effects may be caused accidentally, or may be caused by purposeful hacking. A corrupted (or co-opted) driver might cause additional problems outside the computer, such as causing a phone line or wireless channel to be used, operating an external peripheral, or deleting important data.
Because the operating parameters of the physical layer, which control the operation of the transmitter of the mobile station, are controlled by the protocol layer using software, it may be possible for a computer program or virus to take control of the mobile station and cause it to accidentally or purposefully transmit outside of its assigned time slot. A wireless communications network, such as a cellular network, relies on a shared infrastructure. A mobile station must adhere to the ‘rules of the road’ or it may cause interference on the network.
If certain functions of the mobile station are controlled in software, a programmer may determine how the GSM control frames are decoded and how the transmitter module is triggered. A virus may then be written and spread over the network to infiltrate the software-based mobile stations. Then, on a particular time and date, the virus could take direct control of the mobile station and transmit continuously or intermittently and inundate the base stations and other mobile units with random frequencies and full power. Such a virus design could enable and disable at random times to avoid detection, robbing the air-time supplier of some or all of his available bandwidth and may even cause a complete shutdown of the network. Such an attack may take only a few affected devices (i.e., as few as one) per cell to disable the cell completely.
The security problems associated with mobile stations operating in a shared infrastructure may be segregated into three levels of severity: tamper-proof, non-tamperproof, and class break. First, a hardware/firmware implementation (such as a cell-phone) is the hardest with which to tamper, because each device must be acquired individually and modified (i.e., tamper-proof). On the other hand, a software-based solution is easier to tamper with, as a hacker can concentrate on a software-only debugger environment (i.e., non-tamper-proof). Finally, a system with the ability to be tampered with that is similar on all systems and allows the tampering to be distributed to a large number of systems of the same type is susceptible to a ‘class-break.’
A software wireless modem is susceptible not only to a class-break, but also it is among those devices whose code may be accessed from the same layer as IP (internet protocol) or another portable code access mechanism. Many software wireless modems may be integrated into computers coupled to networks or the Internet. Such an arrangement increases the susceptibility of the software to being tampered with and controlled.
Communication devices implementing other communications protocols using software may also be susceptible to some of the problems identified above, but to differing degrees and levels of consequence. For example, software drivers for communication devices using copper subscriber lines, such voice band modems (V.90), asymmetric digital subscriber line (DSL) modems, home phone line networks (HomePNA), etc., may be attacked, resulting in the subscriber line being disabled or improperly used. For example, a group of infected software modems may be used in a denial of service attack to continuously place calls to a predetermined number and overwhelm the destination. The software modem could also be used to prevent outgoing or incoming calls on the subscriber line or disrupt HomePNA traffic. Other wireless communication devices implemented in software, such as wireless network devices, could also be commandeered to disrupt traffic on the wireless network.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.